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I currently have an indirect fired 55 galon water tank. The house is pretty big and the former owner had a second, power vented water heater on the other side of the house. It seems he also had the two water heaters working the whole house Can I use a tankless water heater for the second water heater? Do I need to cut off the flow so the each heater only sends hot water to certain fixtures?

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  • What's heating the indirect tank, and why are you interested in tankless? – ThreePhaseEel Mar 24 '19 at 21:06
  • Oil fed boiler heats the indirect. I am interested in tankless as it seems from my research they ill last longer, and I have found some that would be cheaper than the power vented water heaters. There is a propane line in place to where the second water heater would go. – Sam Mar 24 '19 at 21:21
  • I'd be more inclined to switch the boiler up to a modern gas-fired mod-con and abandon the old oil-fired setup, oil tank and all, than to switch to a gas tankless for hot water heating; you'd get more bang for your buck that way, I'm sure – ThreePhaseEel Mar 24 '19 at 21:21
  • Hello, and welcome to Home Improvement. If you give us a few more details, then @ThreePhaseEel 's comment will likely make a good answer for you. – Daniel Griscom Mar 24 '19 at 23:18
  • Thanks all. The boiler is only 7 years old and its Buderus, so it's very good quality. I do not have natural gas, I am in a very rural area. I could switch to propane. It's a big old farm house and there are so many other pressing needs that replacing the boiler is not a feasible option at this time. I think my best bet is to install the second heater and have each heater dedicated to certain fixtures. My main curiosity is how the two different types of heaters would work if they were connected to each other the way the existing hooks up are set up now. – Sam Mar 26 '19 at 14:04
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Three Phase Eel's comment is valid. Look at redoing the whole system. If you have natural gas in your area look at hooking up to it.

The rest of this is off topic to your original question, but I think relates enough to TPE's comment to be worth posting.

Gotchas: Heating units were often oversized for the building. My contractor sold me a 90,000 BTU/hr boiler that rarely runs more than 2 minutes in 15 even in mid winter. I have since realized that I could heat my house with a demand water heater.

Sanity check for boiler operation:

  • Measure it's current duty cycle. This takes a lot of patience, or you can put something like a kill-a-watt meter on it. Usually heating appliaces are hard wired, so you would have to modify it to use a plug for the Kill-a-watt.

  • (If you find that it draws 300 watts for the blower, then it uses .3 kWh per hour of operation. If it uses 1.2 kWh per day, then it it ran 3 hours. 3/24 = 12.5% duty cycle)

  • Now find the difference between the temp you want inside (70), and the average temp outside this week. Suppose that is 25 degrees.

  • Look up what your coldest expected temp in winter is. Easiest way is to look up your plant hardiness zone, then look up the temperature definition for that zone. Suppose it's zone 3 which once in 10 years gets -40.

  • Currently it takes a 12.5% duty cycle to keep your house 25 degrees warmer than outside.

  • In your worst reasonable winter it's 70 - -40 = 110 = 4.4 times your current difference. So your present unit should have been running 4.4 times 3 hours or 13.2 hours on the worst weather you are likely to have.

    • That would mean that your previous unit was oversized by almost a factor of 2.

It's probably worse than that. In cold weather we are better about closing doors and windows.

So by doing this you can 'right size' your replacement unit.


This still is a terrible way to do things, as even if you size it perfectly to your worst case, it's underused most of the time. The best system I've seen for this was a school that had 5 small gas boilers. Each was about 200,000 BTU/hr. Each one had an 'on' point 2 degrees differerent. So one boiler would turn on at a temp of 140F and off at 150. The next would turn on at 138 and turn off at 152. The third would be at 136 and 154. During most of the year boilers 2 and 3 would sit there. #1 would carry the load.

The other two were set up similarly for domestic hot water, but at lower temps.

They were also set up on quick connects. If we had trouble with a unit, we could disconnect it in about 10 minutes, put it in a pickup, drive it to town, and they could fix it, without our paying $120/hour for their time driving.

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    Those three school boilers - it is good practice to rotate the lead boiler, so #1 does not get older than the others... – Solar Mike Mar 25 '19 at 5:30
  • They didn't do that with the boilers. The main heating water circulation pumps, however they did. Was this box full of relays that alternated between two 5 HP pumps. It had a front panel: Off, A, B, Both. #3 heating boiler was considered a 'hot spare' In my time on maintenance it came on only when the outside temps were below -35, and there was a wind blowing. – Sherwood Botsford Mar 26 '19 at 14:35
  • I went to a large dairy that had 5 big boilers (7 feet in diameter and 15ft long) and they rotated the lead boiler each week - so that the hours in use was "evened" out - not exactly but close enough... – Solar Mike Mar 26 '19 at 14:39
  • The drawback to wear-leveling is that when one fails due to age/use, the others won't be far behind, thus incurring relatively huge replacement costs. By allowing #1 to wear out well ahead of the others, it can be replaced, then put in position #3, bumping the old #2 to #1 and the old #3 to #2. This spreads the time of replacement cost, making life easier on the checkbook. Of course, the newer boiler is likely more efficient than any of the old ones, so a rotation may not be the best method, either. Use the most efficient most of the time. – FreeMan Sep 18 '20 at 13:42
  • The most extreme version of this I've run into: Boiler makeup feed for a 500 MW plant: Three pumps: Normally Alternated A,B. If either failed, would alternated between the remaining one and C. D was on a pallet near by. They regularly had drills shutting a manual valve to force a switch over, then replacing the pump. E was in the storeroom. F was back at the factory getting re-specced. They really didn't want to lose water to the boiler. – Sherwood Botsford Oct 3 '20 at 3:10
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Yes. There is no problem with having 2 separate water heating appliances. There's also no problem in them being powered differently.

Tankless is not forever, but you are correct that they CAN last longer. However, if you have a decent plumber around any anode rod water heater can meet and absolutely beat a tankless' lifetime with measured replacement of the anode rod...including your existing water heater, if still sound and/or rust-free.

Also, there is an electric alternative at Home Depot by Westinghouse for the BEST EVER water heater. It's all stainless steel and therefore doesn't need nor use an anode rod. More expensive versions are just buying the warranty on the same exact heater, but they MUST be installed by a pro that follows instructions.

Your final and possibly best option is, to delete the 2nd water heater entirely and install a recirculating pump. This keeps the hot water hot at the distant baths, so there's no waiting. This is really ideal if you don't need the capacity of another tank.

If you do need the or some more capacity, then a bigger boiler tank could work extremely well with the recirculating pump.

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  • The drawback to a recirculating pump is that you're constantly heating water, pumping it through the pipes, then returning it to be reheated. For the 8+ hours/day that nobody's at home and the 8 or so hours at night when everybody's sleeping, this is wasted energy. If the pump is on a timer, though, and only operates when the house is occupied, that can be of benefit. Cost is high, though, with a pump at each fixture and all the return plumbing... – FreeMan Sep 18 '20 at 13:36
  • @FreeMan Why would you need a pump at each fixture? Wouldn’t one pump just pump the hot water in a circulation loop. Then each fixture could tap into that loop. – Lee Sam Oct 18 '20 at 20:38
  • @LeeSam assuming that all hot water runs from the heater, past each tap, to the last one in the line, then yes, that would work. If the house is plumbed in PEX with a home-run for each fixture, then each will need its own pump. If the tank is in the middle of the house with a run heading in each direction, then there would need to be 2 or more (one for each line heading out of the heater). – FreeMan Oct 19 '20 at 12:42
  • @FreeMan I guess it depends on how far the overall distance is to the farthest fixture. I’ve only seen one. – Lee Sam Oct 19 '20 at 15:04

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